Electron discharge device



March 8, 1938. l. woLFF 27,110,564

ELECTHON DISCHARGE DEVICE Filed Feb. 28, 1936 'ORI/241,501 JrUfgfWO/f Patented Mar. 8, 1938 guasti PEENT @t'rih ELECTRON DISCHARGE DEVCE Irving Wold, Merchantville, N. .,l., assignorlto Radio Corporation of America, a corporation of Delaware Application February 28, 1936, Serial No. 66,198

3 Claims.

My invention relates to ultra high frequency oscillators of the magnetron or Barkhausen- Kurz type. More specifically my invention deals with means for preventing electronic bombard- 5 ment of the cathodes of such oscillators.

The term electronic oscillators may be applied to vacuum tube devices in which the electron transit times are of appreciable duration with respect to the periods of associated circuits. In

particular cases, the period of oscillation is determined by the electron transit time. Unstable oscillations have characterized thev operation of electronic oscillators of the magnetron or Barkhausen-Kurz type. It was previously believed that the anode became highly heated and radiated this heat to the cathode. The increased heating of the cathode increased the anode current and thereby varied the operation of the oscillator. 'An attempt to overcome the diiculty by water cooling the anode has proven insu-icient. Current limiters in the anode circuit have been partially successful in stabilizing ultra high frequency oscillators.

The real primary difficulty is that some of the electrons which leave the cathode, either fail to reach the anode, or reach the anode and liberate secondary electrons which oscillate back and forth between anode and cathode. These free electrons under the iniiuence of synchronous alternating electrostatic forces acquire high velocity. Many of these free electrons bombard the cathode and liberate substantial amounts of energy. Thus the cathode is. unduly heated.

Control of. anode current will not completely solve this problem because the free electrons may still bombard the cathode. Likewise, cooling the anode will not stop the bombardment. The principal object of my invention is to provide means .for preventing cathode bombardment. Another object is to form a virtual cathode in an ultra high frequency oscillator of the character described. Another object is in the method of preventing electronic bombardment in an electronic device of the character described.

In the accompanying drawing, Figure I is a circuit diagram of a magnetron oscillator embodying my invention, and

Figure II is a circuit diagram of a Barkhausen- Kurz oscillator including my invention. In Figure I is shown an evacuated envelope l and a cathode 3 mounted on a pair of lead wires 5, l. A grid electrode 9 is arranged to surround the cathode 3. This grid electrode maybe of spiral construction, with or without lateral connecting wires, or the grid may befof lattice construction, or parallel wires may be used. In any event the grid should be spaced from the cathode and preferably non-resonant at the operating frequencies of the tube.

The grid may be supported by a pair of lead 5 wires l l. The cathode is energized by abattery I3.

A biasing battery i5 is connected between the grid and cathode to bias the grid negatively with respect to cathode. A pair of cylindrically shaped anodes l'i-IQ are mounted on lead wires 2l-I-23. 10 The lead wires 2%-23 extend through the envelope and form a transmission line. The transmission line is terminated in a dipole antenna 25.

A bridging conductor 2l may be connected across the transmission line to establish a reso- 15 nant circuit including the anodes. The anode battery 2t is connected through the radio frequency choke coil 3i to the said point of the antenna 25. The negative terminal of the anode battery is connected to cathode battery I3. 20

The magnetic field for the magnetron is supplied by an electro-magnet 33. This magnet consists of a suitable core 35 on which is mounted a coil 3l. The coil is energized by a battery 39. The pole pieces M, lit of the magnet are posi- 25 tioned so that the magnetic lines of force surround and are substantially parallel to the cathode.

If the grid il were omitted from the magnetron and the magnetic field reduced to Zero, electrons 30 would be emitted from the cathode.' Almost all of these electrons would travel in straight paths to the anodes. Substantially no electrons would return to the cathode. Such as did return would strike the cathode at very low, or Zero, velocity 35 and would not liberate appreciable energy.

As the magnetic iield is applied, the electrons flow in curved paths, to the anode electrodes. Increasing the eld strength will increase the curvature of the electron paths. An operating 40 characteristic will be reached in which the electrons will follow closed or spiral paths about the cathode. As is well known, with proper adjustment of the circuit constants and magnetic iield,r oscillations will be established in these circuits. 45

Of the electrons traveling closed paths, some will reach the cathode with all of their energy expended. These electrons will not bombard the cathode. I-Iowever, the oscillatory currents and the electronic new to the ancdes may establish 50 relative phases such that electrons gain energy from the alternating circuit both in their trip to the anode and on the return. A simple case in which this occurs is where the electron leaves the cathode while the anode isin the positive 55 phase of the alternating potential and returns to the cathode when the polarity of the anode reverses. These electrons may strike the cathode and give up a very substantial amount of energy.

rIhe cathode bombardment by electrons traveling at high velocities may become so intense thatV the cathode will be destroyed. In any event the cathode bombardment causes undesired variations of anode current. I prevent these undesired effects by surrounding the cathode with'a grid. VA negative potential is applied to the grid with respect to cathode which is sucient to prevent most of the in phase or re-entrant electrons from reaching the cathode. The negative charge on the grid is maintained at a potential which does not prevent emission of electrons -from the cathode. Thus a negatively charged grid prevents electrons from damaging the cathode and forms a virtual cathode intermediate the grid and anode.

In Figure II a Barkhausen-Kurz oscillator has been shown. lIhe essential difference between a Barkhausen-Kurz oscillator and a magnetron is that the former employs a positively polarized grid and omits the' magnetic field of the magn- Y tron. Within an evacuated envelope 5| are mounted a pair of lead wires 53, 55. These lead wires support a cathode 5l. The outer terminals of the lead wires are connected to a battery 59. A second pair of wires 6l, S3 support an inner grid electrode 55. One of these wires 6| extends through the envelope 5l and is connected to the negative terminal of a biasingY battery Bl. The remaining terminal of the biasing battery is joined to the cathode lead wire 55.

At the opposite end of the envelope 5I a second pair of leads Bil-ll and a supporting wire 13 are arranged as follows:V The lead 69 and support 513 carry the outer grid 15, which is preferably concentrically arranged with respect to the inner grid and cathode. A cylindrical anode 1l is concentrically positioned with respect to the other electrodes and is suitably attached to the lead wire ll. rIhe lead wires 69 and H may be used as a transmission line. Blocking capacitors 19, 8l are included in each lead of the transmission line. The transmission line is terminated in a dipole antenna 83.

The anode battery is connected to the cathode and through a radioV frequency choke 81 to the connecting'lead ll. The second grid l5 is biased positively by a battery 89 which is connected to cathode and'through a radio frequency choke Si to the grid lead 69.

In an oscillator of the present type, as shown in Figure II, the theory of oscillation is that electrons emitted from the cathode proceed toward" the anode. The space between cathode and anode is such that electrons require one half Vof the oscillatory period to traverse the space. On the second half period the electrons reverse their path. Some of the electrons will reach the cathode in a phasal relation which permits the cathode to be bombarded. These bombarding electrons convey substantial amounts of energy to the cathode. In this respect the present oscillator and the magnetron are similar.

The linner grid is charged negatively. The Y charge is suticient to repel most electrons which tend to bombard the cathode, but is not so great that electron emission is prevented. The inner grid not only prevents electron bombardment of the cathode but also establishes a virtual cathode between the inner and outer grids.

I have described the operation of the two electronic oscillators which are essentially the same. In each case electronic bombardment of the cathodes is Vprevented by establishing a virtual cathode about a negatively charged grid which surrounds the` actual cathode. Preventing cathode bombardment stabilizes the oscillator and prevents damage `to the cathode. Y Although I have specifically described and referred to electronic oscillators, the term is not intended to exclude the use of an electronic tube for amplifying ultra high frequencies.

I claim as' my invention:

1. An electron discharge device having cathode and anode electrodes, means for imparting Vto the electrons in said device oscillatory movements which free electrons and cause cathode bombardment, a tuned circuit connected to said electrodes, a source of voltage for said electrodes, means positioned between said cathode and anode electrodes for limiting the electronic bombardment of said cathode 'and means for maintaining said last mentioned means sufliciently negative with respect to said cathode under all conditions of operation to limit cathode bombardment.-

2. In an electronic device of the character described a vacuum tube having cathode, and anode electrodes, means for imparting to the electrons in said device oscillatory movements which free electrons and cause cathode bombardment, a tuned circuit connected to said electrodes, a source of voltage for said electrodes, a grid positioned between said cathode and anode electrodes', and Vmeans for biasing said grid continuously negative with respect to said cathode electrode for limiting the bombardment of said cathode by electrons under all conditions of operation.

3. An electron discharge device of the magnetron type having a thermionic cathode, and anode electrodes, means for imparting to said electrons oscillatory movements, which free electrons and cause cathode bombardment, an oscillating circuit connected to said electrodes, a source of voltage for said electrodes, a grid arranged between said cathode and anode electrodes for limiting the bombardment of said cathode by electrons and establishing a virtual cathode between said negative grid and anode electrodes and means for Vmaintaining said grid sufficiently negative under all conditions of operation to thereby limit cathode bombardment. v

4. An electron discharge device having cathode and aV pair of anode electrodes, an oscillating circuit connected between said pair of electrodes, a source of voltage for said electrodes, a magnetic iield whose lines of force are substantially parallel to said cathode, and means positioned between said cathode and anode electrodes for limiting the electronic bombardment of said cathode.

Y 5. An electron discharge device of the Bark-- hausen-Kurz type having a thermionic cathode, a grid positively polarized with respect to said cathode, and anode electrodes, an oscillating circuit connected between said positive grid and anode electrodes, a source of voltage Vfor said electrodes', means positioned between said cathode `and positively charged grid for limiting the electronic bombardment of said cathode and means for maintaining said lastV mentioned means negative with respect to said cathode under all Voperating conditions Vto thereby limit cathode bombardment. Y Y

6. An ultra. high frequency electronic oscillator having a thermionic cathode and a pair of anode electrodes, a `magnetic eld Vwhose lines of force are substantially parallel with said cathode, aV

cathode between said thermionic cathode and.l

anode electrodes wherebyv electronic bombardment of said thermionic cathode is substantially prevented.

7. An ultra high frequency electronic oscillator of the Barkhausen-Kurz type having a thermionic cathode, a grid positively charged with respect to said thermionic cathode, and an anode electrode, a source of electric power for said electrodes, an oscillatory circuit connected between said positive grid and anode electrodes, means for establishing a virtual cathode between said thermionic cathode and said positive grid whereby electronic bombardment of said thermionic cathode is substantially prevented and means for maintaining said last mentioned means sulciently negative with respect to said cathode under all operating conditions to substantially eliminate cathode bombardment.

8. The method of preventing bombardment of a therrnionc cathode by electrons in an ultra high frequency electronic oscillator having thermionic cathode, and anode electrodes, an oscillatory circuit connected thereto, and a grid electrode surrounding and adjacent said thermionic cathode,v which comprises emitting electrons from said thermionic cathode, forcing said electron to traverse paths through said grid electrode and toward said cathode, imparting an oscillatory movement to said electrons, preventing the return of electrons to said cathode by negatively polarizing said grid with respect to said cathode and maintaining said grid sufficiently negative with respect to said cathode under all conditions of operation to substantially eliminate cathode bombardment.

IRV'ING WOLFF. 

